Electron transfer between Donor (D) and Acceptor (A) sites is a key step in the activation of biological processes such as photosynthesis, respiration in tissue, enzyme catalysis and other biochemical reactions in living cells. A detailed and quantitative understanding of all factors which determine the D-A electron transfer rate has a high priority. In this project, we propose investigating several factors which have not been adequately examined previously The theoretical modeling and numerical simulations that will be carried out will be compared with available experimental data or will be used to suggest experiments for specific molecular systems. Specifically, we will investigate the effect of change of the intermediate and side-chain constituents on the electron transfer rate with the use of well-known state-of-the-art quantum simulation packages, such as MOPAC and others. Also, the effect of temporal disorder, i.e., time-dependent fluctuations, of the site-to-site transfer amplitudes due to solvent-molecule interactions will be calculated for specific heme-like molecules. The coupling of the transferring electron with the conformational substrates will be investigate with the use of a theory that includes irreversible random processes. Molecular Dynamics simulations taking account of the long-range Coulomb potential via an Ewald summation method will be carried out to investigate solvent dynamical effects on the electron transfer rates. This work will be used to interpret available experimental data as well as to stimulate experiments on specific biological molecules.